This invention relates to producing shale oil and related materials from a naturally fractured and leached portion of a subterranean oil shale formation of the type encountered in the Piceance Creek Basin in Colorado.
Numerous portions of subterranean oil shale formations of the above type contain substantially impermeable kerogen-containing minerals mixed with water-soluble minerals or heat-sensitive minerals which can be thermally converted to water-soluble materials. A series of patents typified by the T. N. Beard, A. M. Papadopoulos and R. C. Ueber U.S. Pat. Nos. 3,739,851; 3,741,306; 3,753,594; 3,759,328 and 3,759,574 describe processes for recovering shale oil from portions of subterranean oil shale formations which are substantially free of interconnected flow paths. However, where an oil shale formation containing such mixtures of components has been naturally fractured and/or leached, the impermeable kerogen-containing components tend to be surrounded by a network of interconnected flow paths. In such a flow path-permeated formation a hot fluid may spread throughout the flow paths before it transfers enough heat to the kerogen-containing components to pyrolyze a significant amount of the kerogen.
Various situations or processes have been previously encountered or proposed regarding subterranean earth formations in which hot fluids tend to be too quickly dissipated into a network of flow paths. For example, U.S. Pat. No. 2,813,583 describes a fortuitous situation in which the confining of such fluids is aided by nature. The patent describes a hot fluid drive process for recovering oil from the Sprayberry formation in Texas, which formation comprises a laminated shale, limestone, dolomite and sandstone section that contains a network of interconnected fractures through which a hot fluid might be expected to leak into the non-productive layers. But, at least in some such instances, those non-productive layers contain water-swellable clays which tend to swell and plug the fractures. U.S. Pat. No. 2,899,186 describes a process for recovering hydrocarbons by an in situ combustion within a carbonaceous formation having an exposed face or outcrop. A flow-confining barrier is formed along the exposed face by forwardly advancing a combustion front from a line of wells paralleling the face. Reverse combustion is then used to advance the front toward the interior of the formation while allowing a cooling of hot tar to form a flow barrier along the exposed face. U.S. Pat. No. 3,346,044 describes a hot fluid soak process, i.e., injecting hot fluid and then producing fluid from the same well, in an oil shale formation containing a fluid-dissipating network of flow channels. A combustion front is forwardly advanced radially outward, through the flow channels, so that a flow barrier is formed by the subsequent cooling of the heated and displaced oil. The hot fluid soak process is then conducted within the barrier-surrounded portion of the oil shale formation.
Copending U.S. patent application Ser. No. 642,821 filed Dec. 22, 1975 describes a process for producing shale oil from a subterranean oil shale of the type encountered in the Piceance Creek Basin. The oil shale formation components are reacted with hot aqueous alkaline liquid to disaggregate the oil shale matrix, convert at least some kerogen to fluid bitumen and shale oil, and dissolve at least some water-soluble mineral components exposed in or along the walls of an opening or cavity within an otherwise impermeable portion of the oil shale formation. Such a fluid is circulated into and out of the cavity in a manner causing the cavity to enlarge as the shale oil and bitumen are produced. The patent application mentions that such hydrothermal changes in the inorganic portions of the oil shale component can produce a clay or clay-like material.